Volume 106, Number 2, March–April 2001 Journal of Research of the National Institute of Standards and Technology [J. Res. Natl. Inst. Stand. Technol. 106, 391–399 (2001)] Concrete Mixing Methods and Concrete Mixers: State of the Art Volume 106 Number 2 March–April 2001 Chiara F. Ferraris As for all materials, the performance of ule (volume of concrete needed per hour), concrete is determined by its microstruc- and the cost. Ultimately, the quality of National Institute of Standards and ture. Its microstructure is determined by its the concrete produced determines its per- Technology, composition, its curing conditions, and formance after placement. An important Gaithersburg, MD 20899-8621 also by the mixing method and mixer con- measure of the quality is the homogeneity ditions used to process the concrete. This of the material after mixing. This paper paper gives an overview of the various will review mixing methods in regards to [email protected] types of mixing methods and concrete the quality of the concrete produced. mixers commercially available used by the Some procedures used to determine the ef- concrete industry. There are two main fectiveness of the mixing will be exam- types of mixers used: batch mixers and ined. continuous mixers. Batch mixers are the most common. To determine the mixing Key words: concrete mixers; mixer effi- method best suited for a specific applica- ciency. tion, factors to be considered include: loca- tion of the construction site (distance Accepted: August 30, 2000 from the batching plant), the amount of concrete needed, the construction sched- Available online: http://www.nist.gov/jres 1. Introduction As for all materials, the performance of concrete is mixing methods in regards to the quality of the concrete determined by its microstructure. Its microstructure is produced and some procedures used to determine the determined by its composition, its curing conditions, effectiveness of mixing methods will also be given. and also by the mixing method and mixer conditions To determine the mixing method best suited for a used to process the concrete. The mixing procedure specific application, factors to be considered include includes the type of mixer, the order of introduction of location of the construction site (distance from the the materials into the mixer, and the energy of mixing batching plant), the amount of concrete needed, the con- (duration and power). To control the workability or rhe- struction schedule (volume of concrete needed per ology of the fresh concrete, for example, it is important hour), and the cost. However, the main consideration is to control how the concrete is processed during manu- the quality of the concrete produced. This quality is facture. In this overview, the different mixers commer- determined by the performance of the concrete and by cially available will be presented together with a review the homogeneity of the material after mixing and place- of the mixing methods. Further, the advantages and ment. There should be a methodology to determine the disadvantages of the different mixers and mixing meth- quality of the concrete produced, but only few methods ods and their application will be examined. A review of and only one attempt of standardization were found in 391 Volume 106, Number 2, March–April 2001 Journal of Research of the National Institute of Standards and Technology the literature. The methodology to determine the quality purpose is to lift the materials as the drum rotates. In of the concrete mixed is often referred to as the mea- each rotation, the lifted material drops back into the surement of the efficiency of the mixer. The efficiency mixer at the bottom of the drum and the cycle starts parameters of a mixer are affected by the order in which again. Parameters that can be controlled are the rotation the various constituents of the concrete are introduced speed of the drum and, in certain mixers, the angle of into the mixer, the type of mixer, and the mixing energy inclination of the rotation axis. There are three main (power and duration) used. types of drum mixers: • non-tilting drum; 2. Hardware: the Mixers • reversing drum; • tilting drum. There are two main categories of mixer: batch mixers The non-tilting drum mixer implies that the orienta- and continuous mixers. The first type of mixer produces tion of the drum is fixed. The materials are added at one concrete one batch at a time, while the second type end and discharged at the other (Fig. 2). produces concrete at a constant rate. The first type The reversing drum (Fig. 2) is similar to the non-tilt- needs to be emptied completely after each mixing cycle, ing mixer except that the same opening is used to add cleaned (if possible), and reloaded with the materials for the constituents and to discharge concrete. The drum the next batch of concrete. In the second type, the con- rotates in one direction for mixing and in the opposite stituents are continuously entered at one end as the fresh direction for discharging the concrete. There are two concrete exits the other end. The various designs of each types of blades attached to the inner walls of the drum. type of mixer will now be discussed. One set drags the concrete upwards and toward the center of the mixer when the drum rotates in one direc- tion; the second set of blades pushes the concrete toward 2.1 Batch Mixers the opening when the drum rotates in the other direc- Two main types of batch mixer can be distinguished tion. The blades have a spiral arrangement to obtain the by the orientation of the axis of rotation: horizontal or desired effect for discharge and mixing. Reversing drum inclined (drum mixers) or vertical (pan mixers). The mixers are usually used for batches up to 1 m3 [1]. drum mixers have a drum, with fixed blades, rotating The truck mixers belong to the reversing category of around its axis, while the pan mixers may have either the drum mixers. The driver of the truck can control the blades or the pan rotating around the axis. speed of rotation with a clutch in the cabin. The speed depends on whether the concrete has been well mixed prior to being placed in the truck or whether the truck 2.1.1 Drum Mixers has to do most of the mixing. Typically the speed All the drum mixers have a container with a cross for mixing is 1.57 rad/s (15 rpm), while the transport section similar to that shown in Fig. 1. The blades are of pre-mixed concrete uses only 0.2 rad/s (2 rpm) to attached to the inside of the movable drum. Their main 0.6 rad/s (6 rpm) [1]. In the United States, most Fig. 1. Cross section of drum mixers. 392 Volume 106, Number 2, March–April 2001 Journal of Research of the National Institute of Standards and Technology Fig. 2. Cross section of a non-tilting mixer [1]. ready-mixed concrete is mixed in trucks [2] and not rotation is almost vertical the blades cannot lift the con- pre-mixed in a plant. crete and the concrete is not well mixed. The drum axis In a tilting drum mixer (Fig. 3), the inclination can be usually stays at an angle of about 15Њ from horizontal varied. When the drum is almost horizontal (inclination during mixing. To discharge the concrete the drum is ≈ 0Њ), more energy is provided to the concrete because tilted downwards (Fig. 3) below the horizontal plane. more concrete is lifted to the full diameter of the drum The tilting drum is the most common type of drum before dropping. It is during the drop that the concrete mixer for small batches (less than 0.5 m3) both in the is knitted and mixed. Therefore, the higher the drop, the laboratory and in the field [1]. higher the energy imparted to the concrete. If the axis of Fig. 3. Cross section of a tilting mixer. 393 Volume 106, Number 2, March–April 2001 Journal of Research of the National Institute of Standards and Technology 2.1.2 Pan Mixers and not moving. If the pan is fixed, the scraper must move to push concrete toward the blades. Usually the All pan mixers work on basically the same principle individual moving parts, i.e., the blades, the pan, and the [3]: a cylindrical pan (fixed or rotating) contains the scraper, are independently powered. concrete to be mixed, while one or two sets of blades To discharge the mixer, the pan is usually emptied rotate inside the pan to mix the materials and a blade through a trap on the bottom. For small mixers (less scrapes the wall of the pan. The shapes of the blades and than 20 L or 0.02 m3), the blades are lifted and the pan the axes of rotation vary. Figure 4 shows the different can be removed to empty the mixer. combinations of blade configurations and pan. The other element of the mixer is the scraper. Sometimes the axis of rotation of the blades coincides with the pan axis 2.2 Continuous Mixers (single paddle mixer, Fig. 4a and b). Other pan mixers The second category of mixers is continuous mixers have the axis offset [planetary motion mixer and coun- [4]. As the name indicates, the materials are continu- ter-current motion (Fig. 4d and e)]. In these cases (Fig. ously fed into the mixer at the same rate as the concrete 4d and e), there are two rotations: the blades rotate is discharged. They are usually non-tilting drums with around their axes and around the axis of the pan (arrow screw-type blades rotating in the middle of the drum.